For users who do a lot of writing and editing in textarea fields, this extension is a nice supplement to Firefox's built-in ability for finding and highlighting text on a page. What's missing from Firefox's "Find in Page" (Ctrl-F) tool is the option to replace text that has been found. This extension fills that void. I agree that this extension would be even better if the developer could somehow incorporate Firefox's existing ability directly into the pop-up, so that finding, highlighting and replacing words in a text box could be done from a single interface.
UPDATE (2/23/18): There doesn't seem to be any way to get this extension to work with the latest version of Firefox (FF 57/58, aka Quantum). However, it still works for me in FF 52.6 ESR.